Solid tumor models are being used for devising improved therapeutic scheduling of radiotherapy, chemotherapy, and immunotherapy. Each solid tumor model has properties which make it suitable for particular studies involving the complex tumor-host response, the concomitant immunity of the host, and the dissemination of disease through metastases. The different treatment modalities alone or in different combinations will be studied for their effects at four organizational levels, the molecular, cellular, organ and animal levels, permitting maximum clinical utilization of information obtained from the solid tumor models. To attempt an inclusive definition of the effects of therapeutic agents on solid tumor models, biophysical, biochemical, biomathematical, cell kinetic, hematological, pathological and immunological parameters are being used to analyze changes following therapy. The biological end points used in determining optimum drug doses and optimum time schedules include tumor cell viability, total tibial marrow DNA content or cell number, peripheral blood cell concentrations, tumor volume changes, and host survival. It has been demonstrated that the combined modality therapy can be carried out so as to induce maximum effects on the tumor following the recovery of the host from the previous treatment of combined radiotherapy and chemotherapy. Combined immunotherapy and chemotherapy results indicate that immunotherapy (BCG cell walls) can protect the animal from the toxic effects of chemotherapy. The results to date indicate that it is feasible to design future experiments so as to optimize radiotherapy, chemotherapy and immunotherapy to provide maximum protection and minimum effects on the host while producing maximum effects on the solid tumor models. The quantitative information which will be derived from these studies on animal models should aid the clinician in the optimum sequencing of different treatment modalities and be very useful in our overall national efforts designed to improve the management and treatment of patients with solid tumors.